1. Field of the Invention
The present invention relates generally to optical spectrometry, and more particularly to optical spectrometers for use with distributed arrays of fiber optic Bragg gratings (FBGs), for use in such applications as strain sensing, temperature sensing, and other sensing systems for which distributed FBG arrays may be used.
2. Description of the Related Art
Optical spectrometers analyze light signals by measuring the intensity of the light over a range of wavelengths. Optical spectrometers are often used in conjunction with FBG arrays to detect the various light wavelengths that are reflected by the individual gratings in an FBG array.
An FBG array is a length of optical fiber with reflective gratings written into the fiber at intervals. Typically, the gratings in an array will have various peak reflectance wavelengths (the wavelength at which the grating reflects the maximum amount of light back to the source). The peak reflectance of a grating will change, however, as the grating stretches or contracts, typically due to thermal expansion or mechanical strain. By monitoring these changes, one can monitor how the environment is affecting the FBGs, and thereby deduce such things as temperature or strain at the FBG. The wavelengths of light reflected from an FBG array is therefore an optical signal to be analyzed. Typical optical spectrometers for use in conjunction with FBG arrays in these applications are slow and optically inefficient, requiring strong light signals, and have only moderate wavelength resolution.
Two types of optical spectrometers are generally available: serial spectrometers and parallel spectrometers.
Serial spectrometers scan sequentially through a range of wavelengths. They are inherently slower, and at risk for time-dependent errors, but may be more compact than parallel spectrometers.
However, they are inherently inefficient, because only a tiny portion of the employed wavelength range is collected at any time.
Parallel spectrometers have the advantages of optical collection efficiency (in that a large wavelength band is analyzed at any given time) and speed (i.e., data sampling rate), in that they can analyze a broad range of optical wavelengths simultaneously. A typical spectrometer is shown in U.S. Pat. No. 5,719,672 to Chien et al.
There are several drawbacks to applying conventional parallel spectrometers for use with FBGs. One of the disadvantages is the tradeoff between size and resolution. Another is the relatively poor optical-to-electrical efficiency (electrical signal/photon of input light), dynamic range, and speed that most of them achieve.
Previously, the data from parallel spectrometers has been analyzed by using a simple weighted average, or a simple variation of a simple weighted average, to identify spectral peak positions. This can result in appreciable errors in the presence of noisy data. A better method, preferably one permitting resolution down to 1/100th to 1/1000th of a pixel, and less sensitive to noise, is desired.